Citigroup is reissuing MasterCard credit and debit cards used in the U.K., Russia and Canada, saying they may have become compromised following an unspecified breach of its network.

"Last year, Citibank and our customers were the victims of a third-party business' information breach," the company said Wednesday in a statement. "In mid-February, we detected several hundred fraudulent cash withdrawals in three countries. We are currently reissuing cards, as appropriate, to affected customers."

In an earlier statement, published in media outlets, Citigroup said that the accounts may have been compromised in "previous retailer breaches in the U.S.," and that the company was aware of fraudulent ATM cash withdrawals being made in the U.K., Russia, and Canada. The company did not say how many cards were affected by these breaches.

Citigroup, which does retail banking under the name Citibank, did not provide any details on the retailer breaches that prompted this action, but it said it has blocked PIN-based transactions on some cards in those three countries.

Last week Wal-Mart Stores' Sam's Club members-only retail chain confirmed that it was looking into a possible compromise of its fuel station point of sale system. But no PINs were used in any of the fraudulent transactions reported in this case, which involved about 600 cards, according to Wal-Mart.

News of the Citigroup breach first surfaced over the weekend, when Boing Boing Web site contributor Jake Appelbaum reported that he had been unable to use a Citibank ATM card in Toronto.

After calling Citibank customer service on Saturday night, Appelbaum was told that he would have to return to the U.S. to change his PIN number before the ATM component of his card would be useable again. "They told me by using my ATM card on the Canadian network it automatically locked the ATM portion of my card," he said in an interview Tuesday.

The MasterCard portion of the card continued to work normally, but Appelbaum was left frustrated by the fact that he was unable to access the cash in his bank account as he waits for a reissued card, and that Citibank could not say whether the new card will work in Canada. "I was dumfounded by that," he said. "It was the worst customer service I've ever heard of from a bank."

The IDG News Service is a Network World affiliate.